1. Field of the Invention
The present invention generally relates to a method for measuring lattice defects in a semiconductor, and in particular, to a method for measuring a concentration or density of lattice defects producing low energy unharmonic excitation. The present invention more specifically relates to a method for measuring a concentration or density of interstitial oxygen impurity and/or crystal defects relating to the interstitial oxygen impurity in silicon crystal.
2. Description of the Related Art
As is well known, silicon crystals are widely used for substrates of semiconductor devices such as a very large scale integrated circuit (VLSI). In general, a silicon crystal is required to be as pure as possible. That is, it is desired that the silicon crystal has very few lattice defects. In contrast, there are some cases where lattice defects are positively utilized for particular applications. Useful lattice defects depend on the type of impurities or crystal defects. For example, interstitial oxygen impurity and/or crystal defects relating to the interstitial oxygen impurity (initially precipitated nuclei) are a typical example. The interstitial oxygen impurity and/or the related defects are precipitated and/or grown by a heat treatment process. The precipitation of the interstitial oxygen impurity and/or the related crystal defects are useful for intrinsic gettering (frequently abbreviated as IG), in which the interstitial oxygen impurity and/or the related crystal defects have a function of attracting heavy metal impurities upon fabricating the silicon devices. In the intrinsic gettering process, it is necessary to suitably define the concentration (density) of the interstitial oxygen impurity and/or the related crystal defects and to optimize the corresponding process conditions. Also, it is necessary to precisely validate the concentration of the interstitial oxygen impurity and/or the related crystal defects upon producing silicon crystal or for the quality control of the produced silicon crystal. For these reasons, it is important to measure and know the concentration of the interstitial oxygen impurity and/or the related crystal defects.
Conventionally, the concentration of interstitial oxygen impurity in a silicon crystal, which is one of the lattice defects, is measured by means of an optical measurement such as an infrared absorption method or a far infrared absorption method. These methods utilize a physical phenomenon that the interstitial oxygen impurity causes absorption of intrinsic wavelengths. Therefore, the concentration of the interstitial oxygen impurity is obtainable by the proportional conversion of the intensity of the absorption measured.
Of the absorbed wavelengths intrinsic to the interstitial oxygen impurity in the silicon crystal, an absorption band of a wavelength of 1106 cm.sup.-1 is widely used for the infrared absorption method. At a lower temperature equal to or less than 20.degree. K, an intrinsic absorption in the far infrared band (25-60 m.sup.-1) is available. The accuracy of the far infrared absorption method is thus better than that of the infrared absorption method.
However, with the optical measuring methods described above it gradually becomes more difficult to measure the intensity of the absorbed wavelength, as the concentration of dopants (P, B, As, Sb or the like) included in the silicon crystal increases. In the case in which the silicon crystal is in a lower resistivity range of 0.05-0.005 .OMEGA..multidot.cm (i.e., heavily doped over 0.3 ppm), which is useful for practical use, it is impossible to measure the absorption intensity due to the interstitial oxygen impurity in the heavily doped silicon crystal at not only room temperature but also low temperature. In addition, it is currently impossible to detect crystal defects such as an initially precipitated nuclei relating to the interstitial oxygen impurity. Reasons for the disadvantages of the conventional optical measurement are as follows.
In the case of infrared absorption at room temperature, the strong absorption due to many free carriers caused by the abundant dopants, or the dopants of high concentration, conceals the intrinsic absorption of the interstitial oxygen impurity and/or the related crystal defects. In the case of infrared absorption at a lower temperature (substantially equal to a temperature of liquid helium), there exist many neutral donors and acceptors caused by the abundant dopants. Strong absorption, which occurs when these donors and acceptors are ionized, conceals the intrinsic absorption of the interstitial oxygen impurity and/or the related crystal defects.
In the case of far infrared absorption at room temperature, each of the vibrational quantum levels of oxygen impurity is uniformly excited by the application of heat, independent of the concentration of dopants. Accordingly, the induced absorption and emission are substantially equal to each other, and therefore, no real absorption occurs. In the case of far infrared absorption at a lower temperature, the impurity levels of the dopants form bands resulting from the abundant dopants. Then, the strong far infrared absorption due to the intra-band optical transition in the impurity band conceals the intrinsic far infrared absorption of the interstitial oxygen impurity and/or the related crystal defects.
Also, conventional measuring methods other than optical measuring methods are known for especially determining the concentration of the interstitial oxygen impurity and/or the related crystal defects in the highly doped silicon crystal. Examples of these are (i) a secondary ion mass spectroscopy (generally abbreviated as SIMS), (ii) a radio activation analysis and (iii) a method for carrying out the infrared absorption measuring method after compensating the acceptors by injecting a high energy electron beam.
However, in methods (i) and (ii), all of the oxygen atoms included in the silicon crystal are simply detected as a whole, irrespective of conditions of the oxygen atoms. Therefore, it is impossible to discriminate the interstitial oxygen impurity and/or the related crystal defects from other oxygen atoms and to determine the concentration or density. It should be noted that the interstitial oxygen impurity and/or the related crystal are the most important factors for producing semiconductor devices.
The above method (iii) can discriminate the interstitial oxygen impurity and/or the related crystal defects from other defects. However, there is a possibility that the projection of the high energy electron beam changes the conditions of silicon crystal subject to the measurement. In this regard, method (iii) may be said to be a kind of destructive measurement. In addition, method (iii) is effective only for p-type crystals. Moreover, the measuring work is cumbersome.